When utilizing high speed current limiting circuit interruption to interrupt the circuit current in the early stages of the current waveform, it is important that the movable contact arm size be as small as possible to promote electrodynamic repulsion. A further approach to improving the dynamics of contact arm repulsion is to eliminate the heavy flexible braid conductor that connects the contact arm to the load terminal.
U.S. patent application Ser. No. 327,655 entitled "Molded Case Circuit Breaker Movable Contact Arm Arrangement" describes a "braidless" connection between the movable contact arm and its support which does not require any flexible braid conductor and which is of a small size to promote electrodynamic repulsion. This Application is incorporated herein for reference purposes and should be reviewed for its detailed description of the movable contact arm support. When current limiting circuit interrupters are used within higher ampere rated circuits, a small auxiliary flexible conductor is used to provide a parallel current path to deter pitting and such other corrosive electrical effects from occurring at the contact arm-contact support interface.
One example of a silver impregnated-tungsten carbide contact for circuit interrupters is found within U.S. Pat. No. 3,686,456 which Patent is incorporated herein for reference purposes. When such a contact is welded or brazed to the copper movable contact arm, a flux material containing an acid compound is used to provide a clean interface surface for good electrical and thermal transfer between the silver-tungsten carbide and the copper materials. At the interface between the contact and the contact arm, a eutectic alloy of silver and copper is formed having a melting point lower than that of the copper and the silver.
A further example of a braidless movable contact arm is found in U.S. Pat. No. 4,733,033, which Patent is incorporated herein for reference purposes. This Patent discloses the use of a spring having a planar configuration capable of holding the contact arm against its support posts with sufficient force to maintain electrical contact during overcurrent conditions. When this design is used within higher ampere-rated current limiting industrial circuit breakers, a parallel current path should be connected between the movable contact arms and the contact arm support posts to prevent the occurrence of arcing between the contact arm and the support posts under intense short-circuit overcurrent conditions.
With prior art noncurrent-limiting movable contact arms containing flexible braid conductors, the larger mass of copper material constituting the contact arm provided adequate "heat sink" capacity to the silver-tungsten contacts such that when the contacts are subjected to intense short circuit test conditions, the contacts remain at a temperature lower than the melting temperature of the eutectic alloy. With current-limiting movable contact arms it is important to maintain attachment between the contact and the contact arm after multiple short circuit interruptions. The eutectic alloys, that do not present a problem with larger contact arms because the short circuit interruption temperatures are moderated by the larger "effective" heat capacity of the contact arm, could be problematic within smaller-sized circuit interrupters in that the temperatures generated under extreme short circuit test conditions could momentarily exceed the eutectic melting point.
One purpose of the instant invention accordingly is to provide a current-limiting copper contact arm that does not generate a silver-copper eutectic alloy upon attachment of the silver-impregnated tungsten carbide contact.